It is known that a system for transmitting power from one shaft to another by means of a belt requires the presence of a belt tensioner in order to obtain satisfactory operation, and consequently, proposals have already been made for numerous tensioner devices that include both a metal spring and means for damping the vibrations generated in the belt by cyclic irregularities in the running of an engine.
Although such known devices give satisfaction, they are generally complex in structure and thus bulky and expensive.
A simplified and more compact device is described in French patent application No. 89 03512 published under the number FR 2 644 541. That device includes a tensioning wheel that co-operates with the belt, a return spring, a damping device that includes a first variable volume chamber situated inside the return spring covered in a rubber sheath, and a second variable volume chamber situated in line with the spring, the two chambers communicating with each other via a check-valve to obtain damping. The space occupied by the second chamber is not negligible, and in addition the spring relaxes when belt tension diminishes, thereby further increasing the space occupied by that device.
In addition, modern engines are putting constraints of ever increasing severity on belts such that the damping obtained using the device described in the above-mentioned patent application is insufficient since the pressure acceptable in the first chamber is limited by the strength of the rubber sheath covering the spring.
U.S. Pat. No. 4,790,801 describes a hydraulic belt tensioner comprising a cylinder and a piston forming an internal chamber of variable volume and an external chamber coaxial with the inner chamber. The inner chamber and the outer chamber are interconnected by a large non-return ball valve. When the piston rises in the cylinder, thereby increasing the volume of the inner chamber, oil is sucked from the outer chamber, through the valve and into the cylinder. The large dimensions of the valve give rise to little damping. When the piston moves down inside the cylinder, it causes the valve to close, and a large amount of damping is obtained by a throttled flow of oil between the piston and the cylinder.
Unfortunately, it is extremely difficult to calibrate such a flow which, in addition, varies both with operating temperature and with the amount of wear that the device has suffered.